The invention relates to a hydraulic power steering system having a servo valve which is arranged between the delivery and suction side of a hydraulic pump, is open in the central or normal position, and controls the pressure to at least one connection of a hydraulic servomotor. When an operating element of the power steering system is actuated, the hydraulic servomotor serves to generate a servo positioning force which facilitates the adjustment of the operating element. The hydraulic power steering system further has a controllable electric drive for the hydraulic pump.
Power steering systems are generally common in motor vehicles. The servo valve or the servo valve arrangement generally has two parallel throttle routes leading from the delivery side to the suction side of the hydraulic pump and having in each case an input throttle and an output throttle. Both the input throttle and output throttle assume an open position as long as no actuating forces, or only very little actuating forces, are or have to be applied manually at the steering wheel arranged as an operating element. Arranged between the input and the output throttle of each throttle route is a connection for one side of a servomotor which can be actuated in two directions. Pressure equilibrium occurs between these connections as long as the servo valve or the servo valve arrangement is in the central or normal position. As soon as a greater manual setting force acts on the steering wheel, the two throttle is adjusted in the closing direction with simultaneous another in such a way that, in the one throttle route, the input throttle is opened with simulatenously increased throttling of the output throttle while, in the other throttle route, the input throttle is adjusted in the closing direction with simultaneous opening of the output throttle. A greater or lesser pressure difference thus occurs in one direction or the other. This pressure difference depends on the direction of actuation of the steering wheel. In this manner, the servomotor can then facilitate the manual adjustment or control of the steering wheel.
In motor vehicles, the hydraulic pump of the power steering system is usually driven by the vehicle engine. An essentially constant hydraulic flow through the servo valve or the servo valve arrangement is guaranteed by means of an additional hydraulic flow control. This is true even at changing rotary speeds of the vehicle engine and correspondingly changing delivery flow from the pump. In such cases, a relatively high power is also required for the power steering system even when no servo positioning force is to be generated.
In principle it is also known, for example from German Patent document DE 36 22 217 A1, to drive the hydraulic pump electrically. This has the advantage that the drive of the pump can take place independently of the rotary speed of the vehicle engine. In this case, a control of the drive output matching the requirement can be achieved in that the electric motor is controlled in dependence on the hydraulic pressure on the delivery side of the hydraulic pump. This is because, in a servo valve arrangement which is open in the central or normal position, a comparatively high pressure only occurs on the delivery side of the hydraulic pump when the valve arrangement moves out of the central or normal position. Correspondingly, the pump drive can be controlled via a pressure sensor or the like.
It is known from German Patent document DE-A 26 39 853 to control the electric drive of the hydraulic pump of a power steering system in dependence on the turning movements of the steering wheel. The turning movements are recorded in a contactless manner using a rotary position transducer. The electric drive is switched-off when the steering wheel remains steady.
According to German Patent document DE-A 29 15 890, the electric drive of a hydraulic pump of a power steering system is controlled in dependence on the moment of manual force to be overcome at the steering wheel. For this purpose, a torque measuring member is arranged in the steering shaft.
A similar arrangement, in principle, is described in German Patent document DE 34 35 544 A1. Additionally, German Patent document DE 34 35 544 A1 provides for a servo valve arrangement to be dispensed with and, instead, for a pump to be provided. The pump can be driven in two conveying directions. Via the pump, depending on the requirement, one side or the other side of the servomotor can be pressurized.
German Patent document DE 42 08 004 A1 shows the possibility of combining the electric motor, pump and hydraulic reservoir of a power steering system to form a unit which is ready for installation.
Finally, hydraulic power steering systems are known in principle, in which the servo valve or the servo valve arrangement is closed as long as no servo positioning force is required. In this arrangement, the hydraulic pump can be controlled in dependence on the charging state of a hydraulic pressure accumulator which is provided on the input side of the servo valve or the servo valve arrangement. As soon as the steering wheel has to be controlled manually with a specific effort, the servo valve or the servo valve arrangement opens completely or partly, in order to generate a pressure difference. The pressure difference serves to generate the servo positioning force, at the connections of the servomotor. In these systems, the power requirement can indeed be considerably reduced because, in principle, power is only required for the drive of the pump when a servo positioning force is to be generated. However, it has proved to be difficult to achieve satisfactory control characteristics with servo valves or servo valve arrangements which are normally closed
There is therefore needed an advantageous design concept for power steering systems of the above-described type.
These needs are met according to the present invention in that the electric pump drive can be controlled with a mechanically actuable electric switch arrangement whose actuating members can be controlled by movements of mechanical parts of the power steering system.
The present invention is based on the general concept that the entire mechanism of a power steering system has to be adjusted during assembly so that the mechanical elements serving to actuate the electric switch arrangement can likewise be adjusted during this assembly adjustment without any appreciable outlay, or can even inevitably be adjusted.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.